CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-14-2071-2018Climate evolution across the Mid-Brunhes TransitionClimate evolution across the Mid-Brunhes TransitionBarthAaron M.abarth@ehc.eduClarkPeter U.BillNicholas S.HeFengPisiasNicklas G.1Department of Geography and Earth and Environmental Sciences, Emory and Henry College, Emory, VA 24327, USA2College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA3Department of Geoscience, University of Wisconsin – Madison, Madison, WI 53706, USA4Center for Climatic Research, Nelson Institute for Environmental Studies, University of Wisconsin – Madison, Madison, WI 53706, USAAaron M. Barth (abarth@ehc.edu)21December201814122071208711March201827March20185November201826November2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://www.clim-past.net/14/2071/2018/cp-14-2071-2018.htmlThe full text article is available as a PDF file from https://www.clim-past.net/14/2071/2018/cp-14-2071-2018.pdf

The Mid-Brunhes Transition (MBT) began ∼ 430 ka with an increase in
the amplitude of the 100 kyr climate cycles of the past 800 000 years. The
MBT has been identified in ice-core records, which indicate interglaciations
became warmer with higher atmospheric CO2 levels after the MBT, and
benthic oxygen isotope (δ18O) records, which suggest that
post-MBT interglaciations had higher sea levels and warmer temperatures than
pre-MBT interglaciations. It remains unclear, however, whether the MBT was a
globally synchronous phenomenon that included other components of the climate
system. Here, we further characterize changes in the climate system across
the MBT through statistical analyses of ice-core and δ18O
records as well as sea-surface temperature, benthic carbon isotope, and dust
accumulation records. Our results demonstrate that the MBT was a global event
with a significant increase in climate variance in most components of the
climate system assessed here. However, our results indicate that the onset of
high-amplitude variability in temperature, atmospheric CO2, and sea
level at ∼430 ka was preceded by changes in the carbon cycle, ice
sheets, and monsoon strength during Marine Isotope Stage (MIS) 14 and MIS 13.

Jansen et al. (1986) originally described this change in amplitude of
interglaciations as a singular Mid-Brunhes Event, but Yin (2013) argued that
it is more appropriately considered as a transition between two distinct
climate states, thus referring to it as the Mid-Brunhes Transition (MBT).
The change from low-amplitude to high-amplitude 100 kyr variability at
∼430 ka occurs during an interval of reduced eccentricity and
corresponding precession (Fig. 1), but similar orbital forcing occurred at
times before and after the onset of the MBT with no comparable response,
suggesting that the MBT was an unforced change internal to the climate
system. Mechanisms proposed for the MBT include a latitudinal shift in the
position of the Southern Hemisphere westerlies that increased upwelling of
respired carbon in the post-MBT Southern Ocean (Kemp et
al., 2010) and a change in Antarctic Bottom Water (AABW) formation through
insolation-induced feedbacks on sea ice and surface water density (Yin,
2013). However, several questions remain. (1) How and when was the MBT
expressed in other components of the climate system? (2) Was the MBT a
global or regional transition? (3) Did components expressing a transition
change synchronously? Here, we address these questions by providing a
statistical characterization of changes occurring over the last 800 kyr as
recorded by a variety of paleoclimatic proxies with broad spatial coverage.

Site locations. Map indicating the locations of the cores used in
this research and modern sea-surface temperature values. Each symbol
represents a different proxy record. Diamonds indicate sea-surface temperatures.
Circles indicate benthic ∂13C. Squares indicate dust.

MethodsData collection

We compiled all available published records of sea-surface temperature
(SST), benthic marine carbon isotopes ratios (δ13C), and dust
accumulation (dust) that met our selection criteria and closely represented
a global distribution as attainable (Fig. 2). Each data set has an average
temporal resolution of <5 kyr, does not include any large age gaps,
and spans much or all of the entire time period of consideration to limit
biasing of the younger parts of the record. Lisiecki (2014) placed all of
the δ13C records on the LR04 age model. Published SST records
that were not on the LR04 age model were placed on it in one of two ways. If
the original data had depth and benthic δ18O data, the SST
record was placed on LR04 using the ager script in MATLAB as part of the
ARAND software package (Howell et al., 2006). When only benthic
δ18O records were available, the SST records were placed on
LR04 by selecting corresponding tie points in the δ18O data
series using the AnalySeries version 2.0 software (Paillard et al.,
1996). Because some dust records could not be placed on the LR04 age model,
certain statistical analyses of them (e.g., phase–lag relationships) are
likely not robust, but the overall variance in them is preserved. Each
record was then interpolated to a time step (Δt) of 2 kyr. With each
record having an average resolution <5 kyr, this Δt allows
for the preservation of higher-frequency variability while limiting the
number of interpolated data points.

We used empirical orthogonal function (EOF) analysis to characterize the
dominant modes of variability and robustly demonstrate global and regional
signals of the SST, δ13C, and dust records. We then used
spectral analyses of each resulting principal component (PC) to characterize
their periodicity, phase, and amplitude.

Sea-surface temperatures

We used 11 SST records that span the entire 800 kyr time period and four
additional records that span 8–758 ka. Inclusion of these four shorter
records does not change our conclusions. The SST records cover the Pacific
(n=9), Atlantic (n=5), and Indian (n=1) oceans (Fig. 2, Table 1).
We note that Shakun et al. (2015) reconstructed a global SST stack for the
last 800 kyr using 49 records, but only seven of these spanned the entire
800 kyr. Comparison of our SST PC1 based on 15 records to the Shakun SST
stack shows excellent agreement (Fig. S1 in the Supplement).

Carbon isotopes (δ13C)

We analyzed the global set of δ13C records compiled by Lisiecki (2014) (n=26; Fig. 2), and separately analyzed the records in the
Atlantic (n=14) and the Pacific (n=4) basins, thus distinguishing
between the dominant water masses within each basin and removing the muting
effect of the more negative Pacific values on the more positive Atlantic.
Similar to SSTs, Lisiecki (2014) reconstructed a global δ13C
stack for the last 3 Myr using 46 records, but only 18 of these spanned the
last 800 kyr. Comparison of our δ13C PC1 to the Lisiecki
δ13C stack shows excellent agreement (Fig. S2).

We then looked at regional and depth stacks of the δ13C records
in the Atlantic basin to characterize changes in the dominant water masses
on orbital timescales. Regional stacks were broken into North Atlantic
(>20∘ N; n=4), equatorial Atlantic
(20∘ S to 20∘ N; n=14), and South
Atlantic (>20∘ S; n=8). We also created
stacks for the deep North Atlantic (depth >2000 m; n=4) and
intermediate North Atlantic (depth <2000 m; n=3). All included
records were averaged to create the stack and each stacked record was
interpolated to a 2 kyr time step. Stacking improves the signal-to-noise
ratio of the δ13C records, making regional stacks useful in
identifying circulation changes and comparing circulation responses with
other climate records (Lisiecki, 2014).

Data compilation. All data sets are used in these analyses with
associated locations, proxy type, references, and digital object identifier
when available.

We analyzed eight proxy records of dust that span the entire 800 kyr time
period, and then separated them by hemisphere (Northern Hemisphere had three;
Southern Hemisphere had five) to characterize hemispheric differences (Fig. 2). The various proxies
for dust include Fe mass accumulation rates, weight percent of terrigenous
material and Fe, flux of lithogenic grains, and grain size analysis. We
standardized each record before analysis to account for these various proxy
types and their differing range in values, thus allowing for comparison of
their relative amplitudes of variation.

Empirical orthogonal function (EOF) analysis

We used EOF analysis to objectively characterize the climate variability
recorded by the proxies across the MBT. Analyses of covariance between the
data were conducted using the EOF script as part of the ARAND software
package (Howell et al., 2006). The results provide both the dominant
variability as a time series (principal component) and a spatial
distribution of variance contribution (factor loadings). The records for SST
and δ13C were kept in their original values of degrees and per
mil, respectively, to preserve the original variance. Dust records were
standardized to a mean value of zero and unit variance so that each record
provided equal weight to the EOF. Statistical significance of all EOFs was
determined through segmented linear regression analysis. All resulting break
points occur on or after the second EOF and are thus considered significant.

Spectral analysis

We used the Blackman–Tukey technique in the ARAND software package for
spectral analysis of each PC (Howell et al., 2006). Analyses were conducted
using all data points within the time interval of interest, boxcar windowing
of the input data, and hamming spectral filter. Multiple tests were conducted
for the 8–800, 450–800, and 8–350 ka time slices. These intervals
characterize the dominant frequency of variability over the entire 800 kyr
record, and for the pre- and post-MBT intervals, respectively. The removal of
the 350–450 ka interval limited the influence of MIS 11, MIS 12, and
Termination V (T5), as these were shown to potentially bias the spectral
power. Furthermore, these selected intervals result in time series of equal
length to limit biasing of longer records. Additional tests were conducted
using wavelet analyses that characterize the change in spectral power as a
time series. Complementary spectral analyses were conducted on CO2
and CH4 records from the European Project for Ice Coring in Antarctica (EPICA) Dome C ice core
(EPICA-community-members, 2004; Jouzel et al., 2007), and benthic
δ18O using the LR04 stack (Lisiecki and Raymo, 2005).
Cross-spectral analyses were conducted for the PCs against mean insolation
values to determine phase and coherency of each PC. Mean insolation values were
calculated for each of the dominant periodicities (eccentricity, obliquity,
and precession) with the data derived from AnalySeries (Laskar et al., 2004;
Paillard et al., 1996).

Variance tests

We used f tests to test for variance changes across the MBT for each
principal component from the EOF analysis as well as for CO2, CH4,
and the LR04 δ18O records. Analyses were conducted in MATLAB
using the vartest2 script. This approach assumes the null hypothesis that the pre-
and post-MBT distributions of the time series of each climate component have
the same normally distributed variance. If the resulting variance values
reject this hypothesis of no statistical difference, then the pre- and
post-MBT time series are determined to have undergone a significant change
in variance across the MBT. We interpret the change in variance to reflect a
change in the amplitude of each climate signal.

ResultsCO2, CH4, and benthic δ18O

Time series of the greenhouse gases CO2 and CH4 and of the LR04
stack of benthic δ18O suggest an increase in their interglacial
values across the MBT (Fig. 1). Spectral analyses of the LR04 stack and
atmospheric CO2 indicate a small post-MBT increase in the 100 kyr band,
whereas results for CH4 indicate a decrease (Fig. S3). All three
records show an increase in the precessional band (19–23 kyr). Variance
tests suggest that δ18O and CO2 have a statistically
significant increase in variance across the MBT, while CH4 variance
decreases (Table S1).

Sea-surface temperatures

EOF analysis of global SSTs over the last 758 kyr identifies two
statistically significant principal components (Fig. 3a). The first and
second principal components (PC1 and PC2, respectively) account for 69 %
of the total variance, with PC1 explaining 49 % alone. While some degree of
regional variability in each record exists, factor loadings indicate that
each record positively contributed to PC1 with a larger contribution coming
from high-latitude records. Thus, PC1 is representative of a global SST
signal. SST PC1 demonstrates a stepwise increase in variance starting at 436 ka, with an increase of interglacial temperatures, while showing no
significant change in the lower limit glacial values, which is one of the
defining characteristics of the MBT. The highest spectral density is in the
100 kyr frequency band throughout the entire time period (Fig. S3d).
Wavelet analysis (Fig. 4a) shows a significant increase in the
100 kyr frequency band at 580 ka that reaches its maximum spectral power during
MIS 11 and persists throughout most of the remaining interval, albeit with
decreasing intensity after ∼250 ka. Variance f tests reveal a
significant increase in amplitude from the pre- to post-MBT SSTs (Table S1).
These results thus confirm that there was a stepwise global transition of
SSTs from lower- to higher-amplitude interglaciations as previously inferred
from individual records.

Principal components. Plots of the first (PC1; blue) and second
(PC2; red) principal components from our EOF analysis of each climate
variable. Percent variance is explained by each PC represented by the numbers
with the corresponding color. (a) Sea-surface temperatures.
(b) Dust records. (c) Global ∂13C.
(d)∂13C of the Atlantic.
(e)∂13C of the Pacific.

Variance calculations on proxies of bottom water temperature (Elderfield et al., 2012) and on the Antarctic EPICA ice-core
deuterium record (EPICA-community-members, 2004), a measure of Antarctic
atmospheric temperature, also indicate statistically significant increases
in variance across the MBT (Table S1). In both proxies, the time series
indicate an increase of interglacial temperature values while showing no
significant change to the lower limit glacial values, similar to PC1 of SSTs
(Fig. 5).

Dust

The EOF analysis of the global dust records identifies two statistically
significant principal components, with PC1 representing 56 % of the total
variance and PC2 15 % (Fig. 3b). All records but the one from the
Chinese Loess Plateau (CLP) reflect increased dust accumulation due to
increased aridity and/or wind strength during glaciations, whereas higher
dust accumulation in the CLP record reflects increased summer Asian monsoon
strength, which is an interglacial signal (Sun and An, 2005).
Accordingly, factor loadings for the dust records are all positive for PC1
except for the CLP.

In contrast to the change in variance seen in temperature, CO2, and
CH4 during MIS 11, variance tests of the dust PC1 suggest a stepwise
increase in variance during MIS 12, with subsequent glaciations having higher
amplitudes (Table S1). Separating the records by hemisphere shows that the
increase in glacial amplitude starting at MIS 12 occurs in the southern PC1
but not in the northern PC1 (Fig. 6). Similarly, the signal during MIS 14
present in the global PC1 is absent in the northern PC1, suggesting that the
northern control on dust accumulation was skipped during that glacial.

Spectral analysis of the global PC1 indicates dominant power in the
100 kyr frequency band that increases in spectral power across the MBT
(Fig. S3b). Furthermore, wavelet analysis of PC1 demonstrates an increase
in the spectral power of the 100 kyr band at ∼600 ka with its
highest power during MIS 11 (Fig. 4b), similar to the SST PC1. The 100 kyr
frequency remains statistically significant throughout the 100–600 ka interval.

δ13C

The first principal component of the global δ13C (δ13CG; PC1) explains 58 % of the total variance (Fig. 3c). EOF
analysis of δ13C records from the Atlantic basin (δ13CATL) yields two statistically significant PCs, with PC1 and PC2
explaining 58 % and 13 % of the total variance, respectively (Fig. 3d). EOF analysis of δ13C records from the Pacific (δ13CPAC) yields one statistically significant principal component
(PC1 is 81 % total variance) (Fig. 4e).

Dust principal components. The first principal components of our
dust analysis for the global (yellow), north (red), and south (blue) records.
Vertical gray boxes highlight specific glacial (dark gray) and interglacial
(light gray) periods. The numbers indicate the associated MIS of each box.

Both the global and Atlantic PC1 exhibit a strong 100 kyr frequency that is
persistent from 680 to 180 ka (Fig. 4c, d). Unlike SST and dust, however,
δ13CG and δ13CATL
demonstrate a stronger 100 kyr power prior to MIS 11 with its highest power
throughout MIS 13 and 12 (510–460 ka). Spectral analysis shows a decrease
in power of the 100 kyr frequency band from pre- to post-MBT (Fig. S3f, g).
Variance tests show that the pre- and post-MBT intervals for
δ13CG and δ13CATL are
statistically different with higher variance during the pre-MBT (Table S1).
Spectral analyses and variance tests of δ13CPAC
PC1 are similar to δ13CG and
δ13CATL PC1s. The only difference between the
three PC1s is that there is less variance recorded in
δ13CPAC (Fig. 3e). We interpret this muted signal
to be a result of three factors: the large size of the Pacific relative to
the Atlantic, less mixing between water mass end members such as the positive
NADW and more negative AABW, and ocean circulation aging the carbon isotopes
over time leading to more homogenized water masses in the Pacific.

Factor loadings for δ13CATL PC1 are all positive,
suggesting that the time series is representative of the entire Atlantic
basin. In contrast, δ13CATL PC2 yields negative values for
all but the intermediate North Atlantic records and does not show strong
100 kyr spectral power. As such, these results suggest that PC2 exhibits the
dominant mode of variability recorded in the benthic δ13C of
North Atlantic waters shallower than 2000 m depth. Curry and Oppo (2005)
show that NADW formation to below ∼2000 m is reduced in the
North Atlantic during glacial times. The sites with positive factor loadings
in PC2 are located at depths <2000 m, and therefore each site
should remain consistently bathed in NADW through glacial–interglacial
cycles. We thus interpret PC2 as a record of changes in the isotopic values
of the North Atlantic carbon reservoir rather than circulation changes.

During MIS 13, all three δ13C PC1s (global, Atlantic, and
Pacific) demonstrate high positive values. This excursion, first recognized
in individual records by Raymo et al. (1997), clearly stands out
relative to other δ13C interglacial values recorded throughout
the last 800 kyr. The MIS 13 excursion is even more apparent when compared
against other proxy records such as atmospheric CO2, SST, and CH4
(Fig. 7). This high-amplitude change in δ13C values is
similar to the changes recorded in other proxies during MIS 11, yet precedes
the MBT by one glacial cycle. Removal of the MIS 13 interval from variance
tests results in no statistical difference in variance before and after the
MBT, suggesting a large effect of the carbon isotope excursion on these
calculations.

Figure 8 shows regional stacks of δ13C from the deep (>2000 m) and intermediate (<2000 m) North Atlantic and the deep South
Atlantic. As discussed, the intermediate North Atlantic (INA) signal is
predominantly controlled by changes in the carbon reservoir over orbital
timescales. In contrast, the deep North Atlantic (DNA) is controlled by
changes in the relative influence of isotopically more positive NADW and
isotopically more negative AABW, as well as any δ13C
changes to the reservoir that feeds the deep basin from shallower and
surficial waters (i.e., INA). Subtracting the INA from the DNA record (i.e.,
depth gradient) removes the influence of reservoir changes, with the residual
time series reflecting only the relative influences of AABW and NADW on the
isotopic values of carbon in the deep North Atlantic. This is supported by
comparing the North Atlantic depth gradient time series against the South
Atlantic stack (Fig. S4). Both time series demonstrate good correlation for
the entire time interval (r2=0.58), but even more striking is the
similarity in δ13C values, with both time series showing
similar variability and range in δ13C space. The isotopic
similarity between the two records suggests adequate removal of reservoir
influences with the North Atlantic depth gradient, thus reflecting changes in
dominant water mass influence (i.e., circulation). We also note that the
correlation between the two records increases starting at MIS 15 (∼530 ka).

Regional
∂13C stacks. Stacked records of benthic
∂13C separated into three regions: intermediate North
Atlantic (orange), deep North Atlantic (red), and deep South Atlantic (blue).
All plots are shown in ∂13C space to highlight different
isotopic values.

MIS 13 and 5e contour plots of ∂13C. Contour
plots of the ∂13C values in the North Atlantic basin for
the interglacials MIS 13 and MIS 5e. Red colors represent more positive,
enriched values. Blue colors represent lower, depleted values. The plot was created
using Ocean Data Viewer.

The depth gradient does not show the prominent MIS 13 excursion that was
present in the original DNA stack (Fig. 8), suggesting that the excursion
is likely due to a change in the carbon reservoir (represented by the INA)
and not related to ocean circulation. Figure 9 shows contour δ13C plots of the Atlantic basin for MIS 13 and MIS 5e. Although there
is some uncertainty in the these plots due to limited spatial coverage, they
show a clear enrichment of the entire basin during MIS 13 relative to
average post-MBT interglacial conditions, as represented here by MIS 5e. The
global and Pacific δ13C PC1s also show the MIS 13 δ13C excursion, suggesting a change in the global carbon reservoir.

Next, we evaluate the latitudinal gradient between the South Atlantic signal
and the DNA signal in order to further assess the relative influence of the
more negative AABW δ13C values on North Atlantic δ13C values (Fig. 10). Lisiecki (2014) interpreted weaker gradients
during glaciations to reflect shoaling of NADW and greater penetration of
AABW, which could result from reduced NADW formation or stronger AABW
formation. Figure 10b shows a stepwise drop in mean values beginning in MIS 12 (∼436 ka), suggesting a weakening of the gradient due to
greater similarity between North Atlantic and South Atlantic glacial and
interglacial δ13C values.

Discussion

Our new analyses demonstrate that there was a statistically significant
increase in variance in atmospheric CO2, Antarctic temperature, global
SSTs, and bottom water temperature at 436 ka. These changes are consistent
with a transition between two distinct climate states associated with
higher-amplitude interglaciations starting with MIS 11, supporting the notion of a
MBT as defined by Yin (2013). The same climate variables mentioned above
also show an increase in spectral power in the 100 kyr frequency band after
the MBT. On the other hand, the dust analyses suggest that the transition to
greater variability was experienced in the Southern Hemisphere in the
glacial periods starting with MIS 12.

MIS 13 carbon isotope excursion

The PC1 of δ13CG shows a strong correlation with the
CO2 record for most of the last 800 kyr (Fig. 7a). The exception is
during MIS 13, when CO2 levels were still at pre-MBT levels, while
δ13CG shows an anomalously high enrichment relative to
other interglacial values. This is further illustrated by δ13C
contour plots showing that the Atlantic basin was enriched in δ13C during MIS 13 relative to the MIS 5e (Fig. 9).

We evaluated records of biologic activity in various locations of the
Atlantic and Pacific oceans to assess potential sources and sinks in the
carbon system during MIS 13. Ba / Fe from the Antarctic zone (AZ) records the
sedimentary concentration of biogenic Ba and is thus a proxy of organic
matter flux to the deep ocean south of the Polar Front (Jaccard et al., 2013), whereas alkenone
concentrations from the Subantarctic zone (SAZ) indicate export productivity
to the deep ocean in the region north of the Polar Front
(Martínez-Garcia et al., 2009). Based on these proxies, Jaccard
et al. (2013) argued that there were two modes of export productivity in the
Southern Ocean (SO), where high/low export occurs in the AZ during
interglaciations/glaciations, and low/high export occurs in the SAZ during
interglaciations/glaciations. They attributed the increase in SAZ export
productivity to iron fertilization from increased dust accumulation in the
SAZ associated with intensified SO westerlies during glacial periods. Our
Southern Hemisphere dust PC1 record supports this hypothesis in showing that
high values of dust accumulation correlate with increased values of SAZ
export productivity over the last 800 kyr (Fig. S5). We note, however,
that the increase in dust starting at MIS 12 does not have an associated
decrease in glacial CO2 values, suggesting that if iron fertilization
contributed to lower CO2 levels, it had an upper limit beyond which
additional dust fluxes had little effect.

The antiphase relationship between export productivity between the SAZ and
AZ requires a mechanism to increase organic matter productivity in the AZ
during interglaciations as suggested by the Ba / Fe signal (Fig. S5c). In
the modern SO, vertical mixing and upwelling drive the delivery of
nutrient-rich waters necessary for biologic activity to the surface ocean.
Wind-driven upwelling is associated with SO westerlies which shift poleward
during interglaciations (Toggweiler et al., 2006). Thus, any
reduction of upwelling would result from a more northerly position or
decrease in strength of the westerlies; a further decrease in nutrient-rich
surface waters in the AZ during glaciations likely resulted from increased
SO stratification (Sigman et al., 2010;
Jaccard et al., 2013). We note, however, that Jaccard et al. (2013) find no
AZ export productivity during MIS 13, whereas all other interglaciations over
the last 800 kyr show some evidence for it (Fig. S5c). This skipped
interglaciation in export productivity suggests some combination of a change
in the position/strength of the SO westerlies or stratification of the AZ
that limited the delivery of nutrient-rich deep waters to the surface as
compared to other interglaciations of the last 800 kyr.

The PC1s of δ13C (global, Atlantic, and Pacific) demonstrate
that the global ocean was enriched in heavy carbon during MIS 13 relative to
any other interglaciation of the last 800 kyr (Fig. 3). In contrast,
atmospheric CO2 concentrations were ∼ 240 ppm during
MIS 13, similar to other pre-MBT interglacial levels (Fig. 1). Ba / Fe
records of organic export productivity from the AZ that acts as a sink for
light carbon indicate no increase during this interglaciation, while Ca/Al
records from the SAZ indicate increased preservation and thus a deeper
lysocline and lower dissolved inorganic carbon (Jaccard et al., 2010). The
question thus becomes the following: if the ocean is heavily enriched in δ13C
during MIS 13 while CO2 and export productivity remained at low levels,
what reservoir contained the isotopically light carbon?

Paleoclimate records from the CLP indicate greater precipitation during MIS 13 relative to the other interglaciations (Liu, 1985; Yin and Guo,
2008). This greater precipitation has been attributed to increased monsoon
activity recognized throughout monsoonal areas of the Northern Hemisphere
and persisting through MIS 15, 14, and 13 (Yin and Guo, 2008;
Guo et al., 2009). Biogenic silica measurements from Lake Baikal exhibit
continuously high terrestrial productivity in central Asia throughout MIS 15
to MIS 13 (Prokopenko et al., 2002), whereas
sea-level reconstructions indicate that ice volume during MIS 14 was
considerably less relative to other glacial maxima of the last 800 kyr
(Fig. 11d) (Elderfield et al., 2012; Shakun et
al., 2015). Thus, the smaller ice sheets of MIS 14 would likely have had a
lesser effect on displacing forested areas of the Northern Hemisphere,
allowing greater terrestrial carbon storage to potentially persist through a
glacial cycle (Harden et al., 1992). We thus suggest that the increased
monsoonal precipitation and smaller ice volume during MIS 14 would have
combined to increase land biomass that continued into MIS 13. The Northern
Hemisphere thus had the potential to store light carbon in the terrestrial
reservoir resulting in the enriched δ13C MIS 13 signal seen in
the ocean basins (Yin and Guo, 2008).

Ocean circulation changes in the Atlantic basin

One explanation for the glacial–interglacial variations in atmospheric
CO2 invokes a dominant role by the Southern Ocean in storing and
releasing dissolved inorganic carbon (DIC) in the deep Southern Ocean, with
deep-ocean sequestration of atmospheric CO2 occurring through decreased
upwelling and vertical mixing of AABW (Sigman et al.,
2010). Expansion of Southern Ocean sea ice can also lower atmospheric
CO2 by insulating upwelled water from the atmosphere, thus reducing
outgassing, and by increasing the volume of AABW and its capacity to hold
DIC (Stephens and Keeling, 2000; Ferrari et al., 2014). According to this
framework, pre-MBT interglaciations with lower CO2 would be associated
with greater sea-ice extent and a larger volume of AABW, whereas post-MBT
interglaciations with higher CO2 suggest reduced sea-ice extent and
AABW volume. Glacial values of CO2 remain relatively constant
throughout the last 800 kyr (Fig. 1), suggesting that the change in
relative AABW volume before and after the MBT only occurred during
interglaciations.

This mechanism is consistent with ice-core evidence for greater sea-ice
extent during pre-MBT interglaciations (Wolff et al., 2006) and with
modeling results that show that interglacial AABW formation decreased after
the MBT through insolation-induced feedbacks on sea ice and surface water
density (Yin, 2013). Moreover, based on the Ba / Fe proxy of
organic matter flux to the deep ocean south of the Polar Front, Jaccard et al. (2013) argued that the deep
Southern Ocean reservoir was larger prior to the MBT.

Our analyses of changes in Atlantic δ13C over the last
800 kyr further support an important role of AABW in causing the post-MBT
increase in interglacial CO2. In particular, the steeper
latitudinal gradient between North and South Atlantic δ13C
records before the MBT reflects greater northward penetration of AABW,
whereas the post-MBT decrease in gradient suggests greater southward
penetration of NADW (Fig. 10b). These gradient changes are further
illustrated by contour plots of average interglacial δ13C
values in the Atlantic which show that prior to the MBT, AABW penetrated
north of the Equator, increasing the δ13C gradient
(Fig. 12a), in contrast to remaining south of the Equator after the MBT,
decreasing the gradient (Fig. 12c). Removal of MIS 13 and its associated
enriched carbon isotope excursion further highlights the greater volume of
AABW in the pre-MBT interglacial Atlantic (Fig. 12b). We note that a record
of the water mass tracer εNd from 6∘ N (Howe
and Piotrowski, 2017) is in good agreement with our North Atlantic regional
δ13C stack (Fig. 10a), with both records suggesting that
changes in volume of the interglacial AABW occurred south of the Equator.
This reorganization of the dominant interglacial water masses in the Atlantic
basin across the MBT, perhaps resulting from insolation-induced feedbacks
(Yin, 2013), would lead to a greater release of deep-ocean CO2
during the post-MBT interglaciations, with corresponding warmer
interglaciations (Fig. 5). An alternative explanation for the observed
decrease in latitudinal gradient could be changes in the isotopic composition
of AABW across this time period. However, modeling results of long-term
carbon fluctuations across this interval suggest that changes in the burial
rate of organic and inorganic carbon caused the δ13C
depletion – the opposite signal necessary to create the increased similarity
between northern- and southern-sourced waters (Hoogakker et al., 2006). Thus,
it is more likely explained by changes in AABW influence north of the
Equator.

Cross-spectral analysis of pre-MBT North and South Atlantic δ13C stacks indicates in-phase coherency between the records at the
eccentricity and obliquity frequencies. Similar tests for the post-MBT
δ13C stacks exhibit coherency at eccentricity, obliquity, and
precession frequencies, with the South Atlantic stack leading the North
Atlantic by ∼23∘ (7 kyr) in eccentricity,
∼18∘ (2 kyr) in obliquity, and
∼36∘ (2 kyr) in precession (Fig. S6). All
phase relationships overlap within uncertainty, suggesting that South
Atlantic δ13C leads North Atlantic δ13C by 2–7 kyr
following the MBT. This lead by the South Atlantic is most apparent during
terminations (Figs. 9, 12) and is most likely related to deglacial
mechanisms for ventilation of respired CO2 from the deep Southern Ocean
such as enhanced wind-driven upwelling or the melting of sea ice in response
to the bipolar seesaw (Cheng et al., 2009).

Conclusions

Using statistical analyses of multiple climate proxies, we have further
characterized the Mid-Brunhes Transition as an increase in interglacial
sea-surface and Antarctic temperatures, atmospheric CO2, and CH4
beginning with MIS 11. At the same time, our new analyses also document a
number of changes in other components of the climate system that began as
early as MIS 14 that suggest a more complex sequence of events prior to the
MBT, although their relationship to the MBT remains unclear. Figure 13
highlights key features in the sequence of events beginning with an increase
in Asian summer monsoon strength during MIS 15 that persisted through MIS 14
and into MIS 13. The strong monsoon strength during MIS 14 is associated
with a weak glaciation, which in combination would have been conducive to a
build-up of Northern Hemisphere land biomass. A continued strong Asian
summer monsoon during MIS 13 associated with greater precipitation would
have further sequestered land biomass and provided a reservoir for light
carbon, resulting in the oceans becoming unusually enriched in δ13C as recorded in the global benthic δ13C carbon isotope
excursion. MIS 12 was associated with the return of large ice sheets,
collapse of the Asian summer monsoon, and the first increase in amplitude of
Southern Hemisphere dust. A decrease in the latitudinal gradient of
interglacial Atlantic δ13C at the MBT suggests a reorganization
of the water masses in the basin and reduction in the size of interglacial
AABW, thus possibly explaining the increase in interglacial values of
atmospheric CO2 with corresponding increases in interglacial SSTs and
CH4. This evidence for a change in AABW is consistent with modeling
results that suggest that the MBT was forced by insolation (Yin, 2013).

Schematic representation of the sequence of events leading to the
Mid-Brunhes Transition. Corresponding Marine Isotope Stages are located on
the left side of each row. Boxes in a row indicate synchronous events.

All data related to this research, including input and analytical output
data, are available at the PANGAEA data publisher (Barth et al., 2018).
Additional data without persistent digital object identifiers used in this
study were retrieved from Bickert et al. (1997), Lisiecki et al. (2008), and
Pisias et al. (1997), and are available upon request.

The supplement related to this article is available online at: https://doi.org/10.5194/cp-14-2071-2018-supplement.

AMB, PUC, and NGP conceived the project. Statistical
tests were conducted by AMB and analyzed by AMB, FH, and NSB. The paper was
written by AMB, PUC, NSB, FH, and NGP.

The authors declare that they have no conflict of
interest.

This article is part of the special issue “Global Challenges
for our Common Future: a paleoscience perspective” – PAGES Young Scientists
Meeting 2017. It is a result of the 3rd Young Scientists Meeting (YSM),
Morillo de Tou, Spain, 7–9 May 2017.